The invention relates to amalgam spheres for introducing mercury into modern energy-saving lamps.
Modern energy-saving lamps of the TFL (tube fluorescent lamp) or CFL type (compact fluorescent lamp) type belong to the group of low-pressure gas discharge lamps. They comprise a gas discharge bulb which is filled with a mixture of mercury vapour and argon and is coated on the inside with a fluorescent luminophore. The ultraviolet radiation emitted by the mercury during operation is converted by the luminophore coating into visible light by means of fluorescence. The lamps are therefore also referred to as fluorescent lamps.
The mercury required for operation of the lamps was in the past introduced as liquid metal into the gas discharge bulbs. However, introduction of the mercury in the form of amalgam spheres into the gas discharge bulbs has been known for a long time. This makes the handling of the toxic mercury easier and increases the accuracy of metering.
U.S. Pat. No. 4,145,634 describes the use of amalgam pellets which contain 36 atom % of indium and, owing to the high mercury content, contain a large proportion of liquid even at room temperature. The pellets therefore tend to conglutinate when they come into contact with one another. This can be prevented by coating the pellets with suitable materials in powder form. Stable metal oxides (titanium oxide, zirconium oxide, silicon dioxide, magnesium oxide and aluminium oxide), graphite, glass powder, phosphors, borax, antimony oxide and metal powders which do not form an amalgam with mercury (aluminium, iron and chromium) are proposed.
WO 94/18692 describes the use of pellets of zinc amalgam containing from 5 to 60% by weight, preferably from 40 to 60% by weight, of mercury. To produce spheroidal amalgam pellets, the process described in U.S. Pat. No. 4,216,178, in which the molten amalgam is broken up into small droplets by means of a vibrationally excited discharge nozzle and cooled in a cooling medium to below the solidification temperature, is employed.
The pellets are not coated as described in WO 94/18692.
To produce amalgam spheres from the melt, the amalgam has to be heated to a temperature at which the amalgam is completely molten. In the case of a zinc amalgam, this is ensured reliably only at a temperature above 420° C. These high processing temperatures result in a high vapour pressure of mercury and make appropriate safety precautions necessary because of the toxicity of mercury.
JP 2000251836 describes the use of amalgam pellets of tin amalgam for the production of fluorescent lamps. The tin amalgam preferably has only a low mercury content with a tin/mercury atom ratio of 90-80:10-20. This corresponds to a mercury content of from 15.8 to 29.7% by weight. JP 2000251836 gives no information as to how spherical pellets are produced from the amalgam.
A disadvantage of the tin amalgam described in JP 2000251836 is the low mercury content. This makes relatively large amalgam spheres necessary if a particular amount of mercury is to be introduced into the discharge lamps. Owing to the increasing miniaturization which is also being sought in the case of energy-saving lamps, this can lead to problems in the construction and manufacture of the lamps.